Conventional image sensors may be fabricated from a complementary metal oxide semiconductor (CMOS) technology or from charge coupled device (CCD) technology. CMOS image sensors include an array of pixels disposed on a substrate, each pixel including a photosensitive element, such as a photodiode, for example. The photosensitive element and a floating diffusion region may be coupled together when an associated transfer transistor is turned on, to transfer the charge from the photosensitive element to the floating diffusion. There may be one or more layers of metal, polysilicon, diffusions, etc., disposed on the substrate as well.
The typical image sensor operates as follows. Light is incident on the micro-lens, which the micro-lens focuses the light onto the photosensitive element through a filter. The photosensitive element converts the light into an electrical signal proportional to the intensity of the incident light. The electrical signal may be coupled to amplification and readout circuitry such as a CMOS transistor to generate an image based on the captured light.
Conventional image sensors suffer from some limitations. In image sensors that use front side illumination (“FSI”), the layers of metal are disposed between micro-lenses and the photosensitive elements. During fabrication of image sensors that use FSI technology, a channel is therefore created through the metal layers for light to travel from the micro-lens to the photosensitive elements. However, internal reflections at the interface between different materials can cause back reflections, which in turn can reflect off the bottom side of the metal layers into adjacent pixels resulting in optical cross-talk.
One solution is to use back side illumination (“BSI”). In image sensors that use BSI, the layers of metal, polysilicon, diffusions, etc., are on one side of the substrate (front side) and the photosensitive elements are exposed to light from the other side of the substrate (backside). Thus, there is no need to create a path through the frontside metal stack to the photosensitive element. Rather, there is an unobstructed direct path from the backside to the photosensitive element. BSI image sensors suffer from limitations as well. For example, as the pixel size of BSI image sensors becomes smaller, it is increasingly difficult to focus incident light onto the photosensitive element. As a result, there can be crosstalk among the pixels. Crosstalk creates undesirable noise in the image sensor.